Shelf structure



FIG. 1 is a perspective view of a shelf structure, showing my new design, the broken lines are for environmental purposes only and form no part of the claimed design;

FIG. 2 is a front elevational view;

FIG. 3 is a side view;

FIG. 4 is a top view; and,

FIG. 5 is a bottom view. 

The ornamental design for a shelf structure, as shown and described. 